1. Field of the Invention
The present invention relates to a method for charging a battery pack used as a power supply for an electric motor of a hybrid vehicle, or the like, which uses both an electric motor and an internal combustion engine together as an actuator of the vehicle.
2. Description of the Related Art
In a hybrid vehicle which uses both an electric motor and an internal combustion engine together as an actuator of the vehicle, a battery pack including a plurality of battery units is used as a power supply for the electric motor. Such a battery pack needs to be contained in a limited space in the vehicle, and therefore, it is necessary to decrease the size and weight of the battery pack. To this end, a thin, plane square-shaped battery unit has been developed. This square-shaped battery unit is formed by thin and hollow rectangular parallelepiped container made of plastic which contains electrolyte and an electrode plate group immersed in the electrolyte in the container. Furthermore, this battery unit has a safety vent for regulating the pressure in the battery unit.
When the internal pressure of the battery unit begins to increase, the safety vent is activated for releasing the pressure in the container. However, when the internal pressure of the battery unit further increases after the safety vent has been activated, the container may be damaged because it is made of plastic.
For example, the internal pressure of the battery unit increases when the battery unit is overcharged/overdischarged, or when the gas absorption capacity of the battery unit is decreased due to low temperature.
Further, in a hybrid vehicle, the temperature, voltage, current, etc., are measured for each of the battery units which form the battery pack so as to estimate the remaining battery capacity (SOC: State Of Charge) of the battery unit; based on the estimated value of the remaining battery capacity, the amount of electric power which can be added into/extracted from the battery pack (i.e., a chargeable/dischargeable range of the battery pack) is calculated: and the battery pack is used such that the electric power is added into/extracted from the battery pack within the chargeable/dischargeable range of the battery pack. In such a control system, for example, charging/discharging of the battery pack is restricted according to the battery temperature, the remaining battery capacity, etc. So long as use of the battery pack is restricted within the chargeable/dischargeable range of the battery pack, it is possible to eliminate the possibility of the supply of electric power from the battery pack suddenly stopping due to a decrease in the remaining battery capacity (SOC), etc. Thus, improved power performance of the hybrid motor vehicle can be achieved. For example, an uncomfortable feeling of a driver of the vehicle can be eliminated, and the drivable distance of the vehicle can be extended.
In the case where a battery pack including a plurality of battery units is used as a power supply for a hybrid vehicle, an electric current is consumed, i.e., the battery pack is discharged while the vehicle is running, whereby the amount of storage in the battery pack is decreased. When electric power is extracted from the battery pack, the electric power is not extracted equally from the respective battery units. That is, after the electric current has been consumed, the amount of storage differs among the respective battery units. The battery pack with decreased electric power is charged with a constant current such that the respective battery units are uniformly charged so as to supposedly have the same amount of storage. In this uniform charging process, the battery pack is charged with a constant current so that each of the battery units is overcharged.
The internal pressure of the battery unit which is being charged begins to linearly increase at the time around when or immediately after the full charge point is reached. When the uniform charging is performed on the battery pack, each of the battery units is overcharged such that the internal pressure of each battery unit increases to a level at which a safety vent is activated for preventing an increase in the internal pressure of each battery unit. Further, when charging is continued, the internal pressure may increase to a level at which the pressure endurance limit of the container is reached. Specifically, in a square-shaped battery made of plastic, there is a high possibility that an increase in the internal pressure of the battery unit causes the pressure endurance limit of the container to be reached.
In a conventional battery pack, the input/output electric power (the electric power which is added into/extracted from the battery pack) is calculated by estimating the remaining battery capacity (SOC). However, in this calculation process, the increase in the internal pressure of the battery pack is not considered. Thus, whether or not the internal pressure of the battery pack is increased cannot be determined by only estimating the remaining battery capacity (SOC). As a result, an increase in the internal pressure may cause the pressure endurance limit of the container to be reached.
According to one aspect of the present invention, a method for charging a battery pack including a plurality of battery units includes: a first step of charging the battery units with a first electric current at a first rate until an internal pressure of at least one of the plurality of battery units begins to increase; and a second step of charging/discharging the battery units with a second electric current which is lower than the first electric current at a second rate which is lower than the first rate after the internal pressure of the at least one of the plurality of battery units has begun to increase.
In one embodiment of the present invention, the second step includes steps of: charging the battery units; and suspending the charging/discharging of the battery units.
In another embodiment of the present invention, the second step includes steps of: charging the battery units; and discharging the battery units.
In still another embodiment of the present invention, the second step includes steps of: charging the battery units; discharging the battery units; and suspending the charging/discharging of the battery units.
In still another embodiment of the present invention, the first rate is 1.6-3.0 C, and the second rate is 0.3-1.5 C.
According to another aspect of the present invention, a method for preventing an increase in an internal pressure of a battery pack including a plurality of battery units includes steps of: detecting a temperature of at least one of the battery units; estimating an internal pressure of the at least one of the battery units; determining, based on the detected temperature of the at least one of the battery units and the estimated internal pressure of the at least one of the battery units, whether or not there is a possibility that a container of the battery unit will reach a pressure endurance limit thereof; and restricting charging/discharging of the battery pack by performing the second step recited in claim 1 when it is determined that there is a possibility that the container reaches the pressure endurance limit thereof.
In one embodiment of the present invention, the determination of whether or not there is a possibility that the container will reach the pressure endurance limit thereof is performed using a map which represents the pressure endurance limit which has been previously determined.
In still another embodiment of the present invention, the estimation of the internal pressure of the at least one of the battery units is performed based on a remaining battery capacity (SOC) of the at least one of the battery units, the temperature of the at least one of the battery units, and an amount of charged/discharged electric current to/from the at least one of the battery units.
In still another embodiment of the present invention, the remaining battery capacity (SOC) of the at least one of the battery units is calculated based on a voltage of the at least one of the battery units, the amount of charged/discharged electric current to/from the at least one of the battery units, and the temperature of the at least one of the battery units.
Thus, the invention described herein makes possible the advantages of (1) providing a method for charging a battery pack which effectively prevents the container from expanding or reaching the pressure endurance limit thereof when battery units included in the battery pack are overcharged in a uniform charging process; and (2) providing a method for preventing an increase in the internal pressure of the battery pack by which the container is prevented from reaching the pressure endurance limit thereof.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.